EP0449642B1 - Anordnung zum Messen der Dicke eines Gewebes - Google Patents

Anordnung zum Messen der Dicke eines Gewebes Download PDF

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Publication number
EP0449642B1
EP0449642B1 EP91302783A EP91302783A EP0449642B1 EP 0449642 B1 EP0449642 B1 EP 0449642B1 EP 91302783 A EP91302783 A EP 91302783A EP 91302783 A EP91302783 A EP 91302783A EP 0449642 B1 EP0449642 B1 EP 0449642B1
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EP
European Patent Office
Prior art keywords
web
ultrasonic
sensor
thickness
transducer
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English (en)
French (fr)
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EP0449642A1 (de
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Eric J. Reber
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Barber Colman Co
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Barber Colman Co
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
    • G01B7/02—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B21/00—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant
    • G01B21/02—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness
    • G01B21/08—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness for measuring thickness

Definitions

  • This invention relates to thickness measuring systems, and more particularly to non-nuclear caliper-type systems capable of accurate determination of the thickness of a running web.
  • Measurement of web thickness is important in many industrial process applications for various purposes such as, for example, process control or quality assurance.
  • Various systems have been devised for measuring the thickness of running webs of material, and often such gauges are mounted such that they can traverse the moving web across its width to provide a profile of web thickness.
  • nuclear gauges usually include a beta source mounted on one side of the web and a detector on the other for detecting emitted radiation not attenuated by the web.
  • Nuclear thickness gauges are preferred in some applications but they have their drawbacks.
  • Among the disadvantages is the reluctance of certain industries to allow use of a source of nuclear radiation.
  • a second feature which is objectionable in some industries is the fact that nuclear detectors primarily measure density, and therefore the thickness estimates they produce are indirect. Accordingly, if the density of the material being measured varies, the accuracy of the thickness measurements will also vary. Pass line variations, i.e., variation in the position of the moving web with respect to the detector, can also be a problem.
  • ultrasonic gauges can be useful in measuring web thickness since they are capable of accurately detecting the distance between the sensor and the surface of the web at which they are directed.
  • the expected variation (even in a high quality traversing arrangement) of the scanning head with respect to the web can introduce a degree of error which completely masks the nominal accuracy of the transducers. For example, if it were desired to scan a web of nominal 100 mil thickness with a 1% accuracy, it would be necessary to provide a traversing mechanism with a worst case positional variation of ⁇ 0.5 mils.
  • U.S. patent 4,311,392 relates to thickness measuring apparatus and proposes a system utilizing both laser optics and an eddy current detector in the same measuring head.
  • the proposed system utilizes laser optics for detecting the position of the front surface of the web, a backing roll for supporting the rear surface of the web, and an eddy current detector for producing a measurement relating to the distance between the head and the surface of the backing roll. Outputs of the two detectors are combined to produce a measure of sheet thickness corrected for variations in positional relationship between the backing roll and the measuring head.
  • Apparent problems with the system can arise from using relatively complex optical systems in an industrial environment and the necessity for keeping the optics clean.
  • optical source and detector be separately positioned to employ substantially different optical paths not only makes the system complex and potentially difficult to align, but the manner in which the complex optical path is associated with the eddy current sensor can render the system subject to inaccuracies due to relatively minor mechanical misalignment.
  • U.S. Patent 4,276,480 relates to various forms of thickness measuring apparatus.
  • Fig. 7 illustrates a two head system for sensing both sides of an unsupported web, but using relatively complex optical paths and indirect sensing of independent reference lines, requiring the use of four optical light paths in all.
  • the complexity would appear to relate not only to setup and maintenance, but would likely also affect the accuracy obtainable from a device constructed as disclosed.
  • a thickness measurement device which includes, in one embodiment, two reflectance sensors (which may be optical or ultrasonic) for detecting the position of the respective sides of the web, and a transmission sensor, preferably an eddy current sensor, for measuring the total distance of the gap between the sensors.
  • the apparatus is said to be capable of achieving accuracies on the order of 1 or 2 microns.
  • the measuring head or heads are mounted in a frame in such a way that they can "move along the frame".
  • the eddy current sensor measures the total gap dimension, which is said to be a constant, while the reflection sensors measure the distance to the respective surfaces of the web, so that a difference calculation determines the thickness of the web.
  • the device is said to be mounted on the frame so that it can be moved relative to the object to be measured at different points.
  • GB 2,217,835 A discloses these features in the preambles of claims 1 and 6.
  • U.S. patent 4,773,760 discloses a procedure and method for measuring the thickness of a film-like or sheet-like web.
  • a pair of measuring heads each have a light source and a receiver, apparently disposed so that the web is illuminated obliquely, causing the transmission and receiving paths to be different.
  • a sensor such as an eddy current sensor senses the relative position between the sensing heads so as to provide a measure of the total dimension of the gap.
  • Processing means performs the computation which attempts to correct thickness measurements generated by the optical devices for changes in the gap.
  • a more detailed object is to provide a web thickness measuring system which utilizes a pair of measuring heads which can be traversed across the web in unison, each of the heads including sensors operating on energy reflected from the web in order to sense the respective surface positions of the web, and at least one of the heads including a sensor operating on a type of energy to which the web is transparent in order to provide information on the positional relationship between the two heads.
  • an object is to increase the accuracy and reliability of a single head web thickness measuring system by utilizing a pair of sensors in the same measuring head whose positional relationship is optimized to produce thickness readings of maximum accuracy.
  • a non-contact caliper-type gauge for measuring the thickness of a running web.
  • the preferred form includes a pair of scanning heads mounted for traverse on opposite sides of the running web to form a gap, the approximate size of which is known.
  • a reflectance transducer is mounted in each of the heads and directed at the gap for measuring the distance between the associated head and the facing surface of the web by means of energy emitted from the transducer and reflected from the facing surface of the web.
  • a transmission transducer is positioned in one of the heads for measuring the total gap dimension between the heads by means of energy transmitted through the web.
  • Processing means determines and outputs web thickness measurements based on the reflectance transducer determinations of the distance to the respective surfaces of the web as modified by the transmission transducer determination of the total gap dimension.
  • the reflectance transducers are ultrasonic transducers which transmit pulses of ultrasonic energy toward the web and detect ultrasonic energy reflected from the web along substantially the same path.
  • the transmission transducer in such preferred embodiment is an eddy current sensor which responds to the metallic or conductive surface of the opposed head to provide a measure of the gap dimension between the heads.
  • the system is easily retrofit on existing equipment and can operate reliably in an industrial environment with little care, due to the highly reliable ultrasonic transducers for detecting the surfaces of the web, and the eddy current transducer which continuously monitors gap dimension.
  • the information produced by the sensors for detecting the opposite surfaces of the moving web is also processed to determine the web passline or deviations from the web passline in order to apply passline corrections when appropriate.
  • the invention provides a system where only a single traversing head is utilized to measure the thickness of a web supported on a metallic backing roll.
  • an ultrasonic transducer mounted in the head directs ultrasonic pulses of energy at the web, detects reflected energy from the web surface, and determines the dimension between a reference in the head and the facing web surface.
  • An eddy current sensor also mounted in the scanning head measures the dimension between a reference in the head and the surface of the metallic backing roll.
  • Output means processes the signals produced by the two transducers to determine a measure of the thickness of the web.
  • the sensors are mounted coaxially with the ultrasonic sensor being central and surrounded by the eddy current sensor to accommodate for slight misalignments in the scanning head while maintaining maximum precision.
  • Fig. 1 shows, in partly schematic perspective, a web caliper measuring system exemplifying the present invention.
  • Fig. 2 illustrates the same embodiment in elevation.
  • a measuring system generally indicated at 10 which includes a pair of measuring heads 12, 13 arranged in opposed relationship across a gap 14 which is intended for passage of a web 16 whose thickness or caliper is to be gauged.
  • the thickness measuring system 10 derives positional information regarding the surfaces 17, 18 of the web 16 by means of energy reflected from those surfaces, and uses that information to determine the web thickness, i.e., the distance between the surfaces 17, 18.
  • ultrasonic transducers are utilized, although in certain cases other types of reflectance devices such as optical devices might be used.
  • each of the measuring heads 12, 13 is provided with an ultrasonic transducer 20, 21, respectively mounted in the measuring head 12, 13 in a predetermined relationship to mounting surfaces 22, 23.
  • the mounting surfaces 22, 23 are preferably substantially planar and mutually facing.
  • Fig. 2 illustrates that the ultrasonic transducers 20, 21 are thus mounted to face opposed surfaces 17, 18 of the web 16.
  • Mounted within the ultrasonic sensors 20, 21 are transducers 20a, 21a, respectively, which emit pulses of ultrasonic energy in a direction substantially normal to the web.
  • the transducers are shown as dotted rectangles 20a, 21a located in the enlarged portion of the transducer housing coupled to the mounting surfaces 22, 23, respectively, by means of tube members 20c, 21c.
  • the tubes 20c, 21c are adapted to provide a predetermined distance between the transducer and the measuring zone; it will be appreciated that the transducers can be mounted in any convenient location, with the pulse reception and timing being arranged to produce a signal which is representative of the distance between a known reference in the measuring head 20, 21, and the facing surface 17, 18, respectively, of the web 16.
  • the location of the reference is unimportant so long as it remains constant for the system.
  • the surfaces 22, 23 are taken as illustrative of the reference, although as described above the reference position can be scaled for a location more convenient to the electronics, when desired.
  • each of the transducers 20a, 21a emits pulses of ultrasonic energy toward the facing surface of the web 16.
  • the same transducer is utilized to sense energy reflected from the facing surface.
  • Signals are coupled by means of respective cables 25, 26 from the ultrasonic transducers to electronic circuit boards 27, 28.
  • Circuitry in the electronic circuit boards 27, 28 controls the generation of pulses, the detection of reflected pulses, and measures the timing between the initiation of transmission of a pulse and the detection of a reflection as a measure of the distance between the reference associated with the particular sensor and the facing surface of the web.
  • the processed signals originating from the respective transducers are coupled via associated cables 30, 31 (see Fig.
  • the processor 32 produces such a result and outputs it to an output means 34 in any convenient format.
  • the output mechanism 34 in Fig. 1 is shown as a display device which can be a visual display, or a recording type display such as a magnetic tape or magnetic disc.
  • the output device can also, of course, take the form of a process monitor or process control computer system which utilizes the data for process condition monitoring, closed loop control of the system which produces the web, quality control monitoring, and the like, all as is well known in this art.
  • the system of Figs. 1 and 2 is capable of deriving information with respect to the distance between references in the two heads and the facing surfaces in the web, and can thus compute a measure of web thickness, but a measure which is not compensated for is variation in positional relationship between the two sensing heads.
  • means are provided for continually monitoring the dimension of the gap between the two transducers in order to accurately produce measurements of web thickness which are corrected for any systematic or non-systematic deviations in the distance between the two measuring heads.
  • Figs. 1 and 2 Digressing for a moment, it is noted in Figs. 1 and 2 that the measuring heads 12, 13 are mounted in a pair of channels 40, 41 which allow traverse of the measuring heads across the width of the web so as to allow production of thickness profiles of the web across its width.
  • the channels 40, 41 and the mounting arrangements for the heads 12, 13 in those channels is illustrated only schematically. Such arrangements are commercially available, and are sometimes referred to in the art as O-frames.
  • O-frames provide mounting for the respective heads and a drive which causes traverse of the heads in unison across the width of a machine, such as a papermaking machine.
  • sensor means are associated with the pair of scanning heads in order to produce a signal related to the actual distance between those heads at any particular point in their traverse, such that the distance information can be combined with thickness measurements taken more or less contemporaneously to produce a corrected measure of web caliper.
  • the distance sensing means includes a transducer 50, preferably an eddy current transducer, which operates on energy to which the web 16 is transparent, and to which the opposed measuring head 13 is not.
  • An eddy current sensor can be conceptualized as a magnetic coil serving as a primary.
  • the position of the movable secondary, in this case the measuring head 13 serves to control the magnitude of the voltages and currents in the primary, which thus are a measure of the distance between the measuring heads.
  • the head 13 is shaped to provide an attribute which is reliably and conveniently sensed by the detector 50.
  • the head 13 when using an eddy current sensor 50, the head 13 is shaped such that a planar surface 23 of conductive metal is disposed in a plane substantially parallel to the eddy current sensor 50, such that the plane of metal as it approaches or moves away from the eddy current sensor 50 is accurately sensed.
  • the arrangement is capable of directly sensing the gap between the two heads, rather than providing an indirect measurement by way of sensing respective reference lines.
  • a cable 52 couples the eddy current sensor to the circuit board 27 such that the electronic circuitry of the circuit board 27 converts the signal to data relating to the distance between the reference planes in the two measuring heads 12, 13.
  • any convenient imaginary reference plane in the system can be utilized so long as the processor 32 takes account of the distance information produced by the ultrasonic sensors on the one hand and the gap sensor on the other to produce information which leaves as a result of the computation only the thickness of the web 16.
  • Fig. 3 illustrates the geometrical relationship between the detectors and web in the system of Figs. 1 and 2.
  • Ultrasonic sensors 20, 21 are shown as facing the web 16.
  • Double-headed arrows 20b, 21b illustrate the paths for ultrasonic energy for the respective transducers. Since the transducers are positioned substantially normal to the facing surface of the web, the path of incidence and the path of reflectance are substantially the same. If it is possible, of course, to utilize separate transducers for transmission and reception in each ultrasonic sensor, and the paths then would obviously be non-coincident. However, Fig. 3 shows the preferred form of the invention in which the paths are coincident.
  • the transducer 20a which is the transmitting and receiving element in the sensor, is caused to emit a pulse of ultrasonic energy toward surface 17 of the web 16.
  • a timer within the circuitry 27 is initiated.
  • the radiation travels toward the web as indicated by the arrow 20b, and is reflected from the surface 17 of the web back toward the transducing element 20a of the sensor 20. Receipt of reflected energy at the transducing element 20a of the sensor 20 halts the timer within the circuitry 27.
  • Transducer 21 and its transducing element 21a associated with a similar path for energy 21b has its own timer on circuit board 28 so that distance d2 is measured in a similar fashion. It is noted that distances d1 and d2 need not, and usually are not, the same, but that they are measured in the same way using independent transducers aimed at the respective surfaces 17, 18 of the web 16.
  • additional means are provided for measuring the total distance d t so that the computer associated with the system can operate subtractively to provide a measure of the thickness of the web corrected for the actual measured distance between the transducer elements at the time the thickness measurement was taken.
  • the distance d t is measured by means of an eddy current transducer 50 fixed with respect to one of the sensor heads 12 and adapted to sense change in position of that head with respect to a reference plane 23 in the opposed sensor 13.
  • the eddy current sensor is shown as producing lines of flux schematically illustrated at 51 which intercept the conductive metallic reference plate 23.
  • the signal produced by the eddy current sensor 50 thus varies in magnitude based on the number of lines of flux cut by the plate 23 and thus by its relative position with respect to the eddy current sensor 50.
  • the signal produced on output line 52 is a direct measure of the distance d t .
  • the signal on line 52 can be scaled to reflect a measure of the variation in head position from a predetermined nominal gap.
  • the signal on line 52 is taken as a measure of gap dimension d t , and such signal can be scaled in the processor 32 (Fig. 1) to provide an input to the above-stated equation 1.
  • the gap dimension d t is used with the distance signals d1, d2 produced by sensors 20, 21, respectively, to produce a direct measure of web thickness.
  • Fig. 5 there is shown a block diagram better illustrating the signal processing aspects of the invention.
  • the diagram is segregated into four sections, with the leftmost section including ultrasonic transducers 20, 21, eddy current transducer 50, and the web 16.
  • Sequential sections of the drawing represent the signal processing components on circuit boards 27, 28, the processor 32, and the output device or display 34.
  • the ultrasonic transducers 20, 21 are coupled to control elements in the respective circuit boards 27, 28 which include time to distance converters 110, 111.
  • the time to distance converters are illustrated as functional elements, but it will be appreciated by those skilled in the art that such elements contain the ultrasonic transmission and reception circuitry as well as the timer.
  • the circuitry functions to transmit an ultrasonic pulse currently with initiation of the period of a timer, then to receive the reflection of ultrasonic energy and thereupon terminate the interval of the timer.
  • the time to distance converters 110, 111 then convert the measured time intervals, knowing the speed of sound in the medium (typically air), to compute the distances between a reference in the ultrasonic transducer and the facing surface of the web 16.
  • the circuitry for controlling an ultrasonic transducer in that fashion is commercially available and is not illustrated in detail, but the blocks 110, 111 are intended to illustrate the performance of those functions.
  • the ultrasonic transducers are configured to transmit a pulse of energy to a calibrating object at a known distance, detect the time interval of transmission and receipt, and use the known distance and measured travel time to determine a reference signal relating to the speed of sound at the temperature to which the device is subjected.
  • Fig. 3 illustrates a small calibrating tab 20d associated with the sensor 20.
  • the tab does not interfere in a substantial way with transmission of energy to the web, but does cause a reflection back toward the sensor.
  • the sensor establishes a time window for receipt of a reflection whose travel time would require travel to about the passline of the web.
  • reflections from the temperature calibrating tab 20d would not be in the window.
  • the time window is adjusted to detect reflections from the calibrating tab 20d, and the computation made based on the known distance and the measured travel time to compute the velocity of the ultrasound pulses in the medium, and to then use that velocity as a temperature compensated signal for calculating d1 or d2, the distances to the web.
  • Temperature compensation is also preferably provided for the eddy current transducer 50.
  • the temperature compensation for the eddy current transducer is represented by a block 115.
  • the temperature compensation for the eddy current transducer is accomplished in an analog fashion by a device having a temperature dependent characteristic opposite to that of the eddy current sensor.
  • Such means are well known in the art. They will not be further described herein except to note that they are preferably implemented in analog fashion by utilization of a temperature dependent voltage source mounted on the eddy current sensor such that the eddy current sensor produces an output signal which is temperature independent.
  • the compensated analog signal is then passed to an analog-to-digital converter 116 which is illustrated as an element of the circuit boards 27, 28, but which can also reside for purposes of economy in the computer 32.
  • the compensated signal can be combined in analog form with analog signals from the ultrasound detectors to produce an analog thickness signal compensated for gap dimension.
  • the analog signal from the eddy current transducer 50 as temperature compensated is utilized to produce a gap dimension d t which is processed with the d1 and the d2 signals to produce a measure of the caliper or thickness of the web.
  • the elements illustrated as making up the processor 32 illustrate the manner in which the signals are combined for producing the web thickness output. It is seen that a block 120 performs the computation of equation 1, i.e., subtracts from the total gap dimension d t the sum of the signals produced by the ultrasonic sensor (d1+d2). In the simplest form of the invention, that thickness signal is output directly to display or processor 34.
  • the system in which the thickness signal produced by block 120 is directly output as a measure of web thickness is currently believed to encompass a majority of applications, and will provide accuracy comparable to that expected from nuclear thickness gauges. However, there are a number of occasions where the web is of a character which will partly interfere with transmission of energy through the web, and those applications are expected to be at least partly sensitive to the position of the passline.
  • the web will partly interfere with the coupling between the eddy current sensor 50 and the opposite head. While the conductive web will not make the system inoperable, it will produce a condition where the thickness measurement produced in block 20 is accurate only when the web is in a predetermined passline position, and is in error in a known manner when the passline varies.
  • the system of the invention provides a convenient means for accommodating a passline variable system in that the ultrasonic transducers 20, 21 produce information which not only relates to the thickness of the web, but also its actual passline.
  • additional components in the processor means 32 are provided for the special case where the system is passline sensitive.
  • a first of those components identified as block 125 in Fig. 5 determines the passline deviation from one of the ultrasonic transducer signals.
  • the block 125 calculates the passline deviation ⁇ PL by comparing a known passline position PL stored in the processor with one of the ultrasonic distance signals d1.
  • the stored measure PL is intended to represent the distance d1 sensed by the ultrasonic transducer 21 when the web is in its preferred passline position.
  • ⁇ PL will be zero.
  • a positive or negative ⁇ PL will be determined.
  • a second block 126 stores the function (such as in a table) relating thickness correction to passline variation.
  • the function is linear
  • Such a correction function is represented by the block 126.
  • a passline deivation ⁇ PL is sensed by the block 125, that deviation is applied to the block 126 to determine a thickness correction ⁇ t w which should be applied to the thickness measurement determined by block 120.
  • a block 128 thereupon determines a corrected thickness measurement t w (corr) which is equal to the sum of the computed thickness measurement plus the correction function determined by block 126.
  • the corrected function is then output to the display or processor 34.
  • passline deviation correction is currently intended as an optional feature, and that most applications are expected to be satisfied with the thickness correction as determined in processing block 120.
  • the additional elements illustrated in Fig. 5 can be added without substantial additional cost (the transducer signals are already available) to yield the necessary accuracy even in those difficult situations. The substantial versatility of the system according to the invention will thus be appreciated.
  • a system in accordance with the invention will provide caliper measurements across the width of a running web.
  • the traverse will be at a nominal speed
  • the signals from the sensors will be sampled at a predetermined rate
  • the sampling times for taking analog signals from the sensors and passing them to the computer are coordinated such that the gap correction provided by the processor relates positionally to the thickness measurements provided by the ultrasonic transducers.
  • the signals from the eddy current sensor and ultrasonic sensors are sampled in such a way that the eddy current sensor is measuring the gap at about the same time that the ultrasonic sensors are measuring the position of the web surfaces to produce continually updated and corrected information during the course of the traverse.
  • Such a system can be configured utilizing an eddy current sensor produced by Kaman, Model 60U to produce a 10 mv per mil signal which can be digitized with 12 bit resolution to produce a digital resolution of 0.24 mils.
  • Kaman, Model 60U eddy current sensor produced by Kaman, Model 60U to produce a 10 mv per mil signal which can be digitized with 12 bit resolution to produce a digital resolution of 0.24 mils.
  • the distance between the sensing heads d t can be measured with substantial accuracy commensurate with the desired total system accuracy.
  • the ultrasonic sensors can be obtained from Contaq Technologies of Bristol, Vermont and are capable of producing about an 0.7 mil resolution for a single reading. For a moving web, adding and averaging single measurements will improve resolutions as a regressive statistical function. For example, four sample averaging yields a resolution of 0.59 mils. Thus, using a scanning rate of 3.3 inches per second (for traverse of the paired heads across the web), and an 0.7 mil resolution for the ultrasonic sensor, and four samples per reading, a 1% resolution of a 53 mil target is achievable. If the scanning rate is slowed to 2.5 inches per second, a 1% resolution of a 43 mil target is achievable.
  • the eddy current sensor 50 is arranged in a donut configuration with an aperture 50a of a size adequate to accommodate the ultrasonic sensor 20.
  • a coaxial sensor arrangement is provided which has the ultrasonic sensor 20 arranged in the center for direct transmission and receipt of reflected energy to and from the web, and the eddy current sensor 50a surrounds the ultrasonic sensor for providing an averaged reading of distance between the two scanning heads. Assuming that the scanning frames are such that the sensors are maintained in substantial face-to-face alignment, the accuracies for distance measurement disclosed above can be achieved. Furthermore, a small degree of misalignment can be tolerated, particularly in view of the coaxial arrangement discussed above.
  • Fig. 4 shows a scanning head 12′ adapted for traverse in a bracket 40′ for sensing the thickness of a web 16′ disposed with a surface 17′ facing the head 12′.
  • the second sensing head is eliminated and instead a backing roll 100 is provided on which the web 16′ is supported.
  • the coaxial sensor arrangement then utilizes the eddy current sensor 50′ to detect the distance to the surface of the roll 100 and thereby a measure of the distance of the non-facing surface 18′ of the web, whereas the ultrasonic sensor 20′ provides a measure of the distance to the facing surface 17′ of the web.
  • a calculation is made in computer 32′ (not shown in Fig.
  • the signals are processed in analog fashion in a summing amplifier, to subtract the distance measured by the ultrasonic sensor 20′ from the distance measured by the eddy current sensor 50′ to provide a measure of the thickness of the web 16′ compensated for traversing variations or other non-systematic variations in distance between the head 12′ and the web 16′.
  • the coaxial arrangement of the sensors is particularly important in this embodiment because any misalignment of the scanning head 12′ will have a minimum impact on the central energy transmission and reflection for ultrasonic sensor 20′ while providing an averaging of the distance measured by the outer concentric eddy current sensor 50′.
  • the eddy current sensor 50′ will provide a distance measurement for the distance to the non-facing surface 18′ of the web which averages at least a portion of the misalignment with respect to the ultrasonic measurement which is taken at the center of the coaxial sensor.
  • the coaxial arrangement will provide more accurate readings of the caliper of the web 16′.
  • the coaxial arrangement illustrated in Fig. 4 has the further advantage of compactness, and the ability to take samples of the distance from the head to the respective surfaces 17′, 18′ of the web at about the same instant in time and for about the same increment of web travel over the roller.
  • the types of sensors utilized in the coaxial arrangement i.e., an ultrasonic sensor for reflected energy and an eddy current sensor for energy to which the web is transparent, provide not only what is believed to be maximum reliability in such a system, but an arrangement which can be easily adapted to an existing machine roller (such as a calendar roll in a papermaking machine) to provide accurate, repeatable and reliable thickness measurements in a retrofit environment at an affordable cost.
  • a pair of scanning heads are provided for utilizing reflected energy from each of the surfaces of the web to determine web thickness.
  • One of the measuring heads is provided with an energy sensing system utilizing energy to which the web is transparent for determining the distance between the heads, thereby to compensate for variation in distance between the heads during traverse across the witdth of a web.
  • Advantage can also be taken of the fact that the reflectance transducers provide passline information to make a passline correction when necessary.
  • advantage is taken of the coaxial arrangement of the reflected and transparent energy sensors to utilize only a single coaxial pair cooperating with a backing roll to produce reasonably precise measurements of web thickness, in the case where a second sensor is impractical or undesirable.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices With Unspecified Measuring Means (AREA)
  • Length Measuring Devices Characterised By Use Of Acoustic Means (AREA)
  • Length Measuring Devices By Optical Means (AREA)
  • Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)

Claims (10)

  1. Anordnung zum Messen der Dicke tw einer laufenden Gewebebahn (16), enthaltend oder bestehend aus der Kombination der folgenden Merkmale:
    einem sich hin- und herbewegenden Abtastkopf (12) mit einem ersten (20) und einem zweiten (50) Sensor, wobei der erste Sensor (20) ein Reflexionssensor ist zur Abgabe von Energie auf die Gewebebahn (16) entlang einer bestimmten Strecke (20b) zwecks Reflexion durch die Gewebebahn und Erfassung der Reflexionen von einer ersten Oberfläche (17) der Gewebebahn (16) entlang im wesentlichen derselben Strecke (20b), wodurch der Abstand d₁ zwischen dem Abtastkopf (12) und der ersten Oberfläche (17) der Gewebebahn (16) bestimmt wird,
    der zweite Sensor (50) ein Transmissionssensor ist zur Beförderung von Energie (51) durch die Gewebebahn (16) und zum Ermitteln eines Bezugsmittels (23, 100), das ein vorher festgelegtes Verhältnis zu einer zweiten Oberfläche (18) der Gewebebahn (16) hat, wodurch der Abstand zwischen dem Abtastkopf (12) und dem Bezugsmittel bestimmt wird,
    einem Computer (32), der auf Signale (30, 31) des ersten (20) und des zweiten (50) Sensors anspricht zur Erzeugung eines Maßes der Dicke tw der Gewebebahn (16),
    dadurch gekennzeichnet, daß
    der zweite Sensor (50) den ersten Sensor (20) koaxial umgibt, der sich hin- und herbewegende Abtastkopf (12) zur kontinuierlichen Bewegung über die Gewebebahn (16) hin mit einer Geschwindigkeit gemessen in der Größenordnung von Zoll pro Sekunde eingerichtet ist, und
    der Computer (32) so eingestellt ist, daß er mindestens etwa fünf Meßwerte der Dicke der Gewebebahn pro Zoll Breite der Gewebebahn erzeugt, während der Abtastkopf (12) sich über die Gewebebahn bewegt.
  2. Kombination gemäß Anspruch 1, worin
    der Reflexionssensor (20) ein Ultraschallsensor ist, der Ultraschallenergie auf die Gewebebahn (16) richtet und Ultraschallreflexionen von der ersten Oberfläche der Gewebebahn entlang im wesentlichen derselben Strecke (20b) erfaßt, und
    der Transmissionssensor (50) ein Wirbelstromsensor und das Bezugsmittel (23, 100) ein leitendes Bezugsmittel ist.
  3. Kombination gemäß Anspruch 2, worin das leitende Bezugsmittel umfaßt oder besteht aus:
    einem zweiten Abtastkopf (13), der gegenüber dem ersten Abtastkopf (12) installiert ist und eine leitende Oberfläche (23) hat, die dieses leitende Bezugsmittel definiert, wobei der Wirbelstromsensor (50) im ersten Abtastkopf (12) so beschaffen ist, daß er den Abstand dt zwischen dem ersten Abtastkopf (12) und der leitenden Oberfläche (23) des zweiten Abtastkopfes (13) abtastet, und
    einem zweiten Ultraschallsensor (21), der im zweiten Abtastkopf (13) installiert ist, um Ultraschallstrahlung entlang einer zweiten, vorgegebenen Strecke (21b) auf die zweite Oberfläche der Gewebebahn zwecks Reflexion zu richten, und die Ultraschallreflexionen von der zweiten Oberfläche (18) der Gewebebahn (16) entlang im wesentlichen derselben zweiten Strecke (21b) zu erfassen.
  4. Kombination gemäß Anspruch 2, worin das leitende Bezugsmittel (23, 100) eine glatte Metallwalze (100) enthält oder daraus besteht, die die zweite Oberfläche (18) der Gewebebahn (16) stützt, wobei der Wirbelstromsensor (50) im ersten Abtastkopf (12) so beschaffen ist, daß er den Abstand dt zur Oberfläche der Metallwalze (100) und somit zu der gestützten, zweiten Oberfläche (18) der Gewebebahn (16) abtasten kann.
  5. Kombination gemaß Anspruch 3, worin der Computer (32) außerdem ein Mittel (125) zur Bestimmung des Laufs der Gewebebahn von mindestens einem der Ultraschallsensoren (20, 21) aufweist, sowie ein Mittel (126) zur Ermittlung eines um Laufabweichungen korrigierten Meßwertes der Dicke der Gewebebahn.
  6. Berührungsfreie Tastlehre zur Messung der Dicke einer laufenden Gewebebahn (16), wobei die Lehre die Kombination der folgenden Merkmale enthält oder daraus besteht:
    einem Paar Abtastköpfe (12, 13), installiert (10) zur Bewegung auf gegenüberliegenden Seiten der Gewebebahn (16), um einen Zwischenraum (14) von in etwa vorher festgelegten Dimensionen dazwischen einzuhalten,
    einem Reflexionswandler (20, 21) in jedem der Köpfe, der auf den Zwischenraum (14) gerichtet ist, um den Abstand (d₁, d₂) zwischen dem betreffenden Kopf (12, 13) und der gegenüberliegenden Oberfläche (17, 18) der Gewebebahn (16) mittels vom Wandler (20, 21) entlang einer bestimmten Strecke (20b, 21b) ausgesendeter und von der gegenüberliegenden Fläche (17, 18) der Gewebebahn (16) reflektierter Energie zur Erfassung entlang im wesentlichen derselben Strecke (20b, 21b)
    einem Transmissionswandler (50), der in einem der Köpfe (12) positioniert ist, um mittels durch die Gewebebahn (16) gesendeter Energie die Gesamtdimension des Zwischenraums (dt) zu messen,
    einem Prozessor (32) zur Bestimmung und Ausgabe der Meßwerte der Dicke der Gewebebahn (dw), basierend auf den vom Reflexionswandler bestimmten Meßwerten des Abstands (d₁, d₂) zu den jeweiligen Oberflächen (17, 18) der Gewebebahn (16), modifiziert durch die vom Transmissionswandler (50) bestimmte Gesamtdimension (dt) des Zwischenraums,
    dadurch gekennzeichnet, daß
    die Abtastköpfe (12, 13) zur kontinuierlichen Bewegung über die Gewebebahn (16) mit einer Geschwindigkeit in der Größenordnung von Zoll pro Sekunde vorgesehen sind, und
    der Prozessor (32) so beschaffen ist, daß er mindestens fünf Meßwerte der Dicke der Gewebebahn (dw) pro Zoll der Breite der Gewebebahn bestimmt, während die Abtastköpfe (12, 13) sich über die Gewebebahn bewegen.
  7. Kombination gemäß Anspruch 6, worin die Reflexionswandler (20, 21) Ultraschallwandler sind, ausgerüstet zur impulsweisen Aussendung von Ultraschallenergie auf die Gewebebahn (16) und Erfassung der von der gegenüberliegenden Oberfläche (17, 18) der Gewebebahn (16) reflektierten Ultraschallenergie, und der Transmissionswandler (50) ein Wirbelstromwandler ist, der in einem der Köpfe (12) installiert ist, und ein leitendes Bezugsmittel (23) im zweiten Kopf (13) zum Abtasten mittels des Wirbelstromwandlers (50), um den Zwischenraum (dt) zwischen den Köpfen (40, 41) zu indizieren.
  8. Kombination gemäß Anspruch 7, worin der Ultraschallwandler (20) und der Wirbelstromwandler (50) in dem einem der Köpfe (12) koaxial angeordnet sind, wobei der Wirbelstromwandler (50) den Ultraschallwandler (20) umgibt.
  9. Kombination gemäß Anspruch 7, worin die Ultraschallwandler (20, 21) Meßergebnisse liefern, die geprüft werden, während der Abtastkopf über die Gewebebahn fährt, und aus einer Vielzahl der geprüften Meßwerte der Ultraschallwandler (20,21) Durchschnittswerte gebildet werden, um die Genauigkeit der Messungen der Bahndicke zu erhöhen.
  10. Kombination gemäß Anspruch 6 oder 7, worin der Prozessor (32) außerdem ein Mittel (125) beinhaltet zur Bestimmung von Abweichungen im Lauf der laufenden Gewebebahn (16) von mindestens einem der Reflexionsumwandler (20, 21),
    sowie ein Mittel (126) zur Korrektur der Meßwerte der Dicke der Gewebebahn bezogen auf Laufabweichungen.
EP91302783A 1990-03-28 1991-03-28 Anordnung zum Messen der Dicke eines Gewebes Expired - Lifetime EP0449642B1 (de)

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US500805 1990-03-28
US07/500,805 US5113358A (en) 1990-03-28 1990-03-28 Web caliper measuring system

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EP0449642A1 (de) 1991-10-02
US5113358A (en) 1992-05-12
DE69105791D1 (de) 1995-01-26
KR910017163A (ko) 1991-11-05
JP2766744B2 (ja) 1998-06-18

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